Thin film semiconductor alloys have gained growing acceptance as a material from which to fabricate electronic devices such as photovoltaic cells, photoresponsive and photoconductive devices, transistors, diodes, integrated circuits, memory arrays and the like. This is because the thin film semiconductor alloys (1) can be manufactured at relatively low cost, (2) possess a wide range of controllable electrical, optical and structural properties and (3) can be deposited to cover relatively large areas.
Recently, considerable effort has been expended to develop systems and processes for depositing thin film semiconductor alloy materials which encompass relatively large areas and which can be doped so as to form p-type and n-type semiconductor alloy layers for the production therefrom of thin film electronic devices, particularly thin film p-n type and p-i-n type photovoltaic devices which would be substantially operatively equivalent or superior to their crystalline counterparts. Among the investigated semiconductor alloy materials of the greatest significance are the silicon, germanium, and silicon-germanium based alloys. Such semiconductor alloys have been the subject of a continuing development effort on the part of the assignee of the instant invention, said alloys being utilized and investigated as possible candidates from which to fabricate thin films of amorphous, microcrystalline, and also polycrystalline material.
As disclosed in U.S. Pat. No. 4,226,898 of Ovshinsky, et al, which patent is assigned to the assignee of the instant invention and the disclosure of which is incorporated herein by reference, fluorine introduced into the silicon alloy semiconductor layers operates to substantially reduce the density of the localized defect states in the energy gap thereof and facilitates the addition of other alloying materials, such as germanium. As a result of introducing fluorine into the host matrix of the semiconductor alloy, the film so produced can have a number of favorable attributes similar to those of single crystalline materials. A fluorinated thin film semiconductor alloy can thereby provide high photoconductivity, increased charge carrier mobility, increased diffusion length of charge carriers, low dark intrinsic electrical conductivity, and, where desired, such alloys can be modified to help shift the Fermi level to provide substantially n- or p-type extrinsic electrical conductivity. Thus, fluorinated thin film semiconductor alloy materials can be fabricated in manner which allows them to act like crystalline materials and be useful in devices, such as, solar cells and current controlling devices including diodes, transistors and the like.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, the aforedescribed, amorphous alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in the following U.S. Patents: U.S. Pat. No. 4,400,409, for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom and U.S. Pat. No. 4,410,588, for Continuous Amorphous Solar Cell Production System; U.S. Pat. No. 4,438,723, for Multiple Chamber Deposition And Isolation System And Method. As disclosed in these patents, a substrate may be continuously advanced through a succession of interconnected, environmentally protected deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device, for instance, of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy. The layers of semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form photoresponsive devices, such as, but not limited to photovoltaic cells which include one or more p-i-n type cells. Note that as used herein the term "p-i-n type" will refer to any sequence of p and n or p, i, and n semiconductor alloy layers. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
The concept of utilizing multiple stacked cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein disclosed utilized p-n junction single crystalline semiconductor devices. Essentially, the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass throught the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. Such tandem cell structures can be economically fabricated in large areas by employing thin film amorphous, microcrystalline, and polycrystalline semiconductor alloy materials, such as the microcrystalline p-doped semiconductor alloy material of the instant invention.
It is now possible to manufacture high quality n-doped and intrinsic thin film semiconductor alloy layers utilizing techniques developed by the assignee of the instant invention. However, the p-doped thin film semiconductor alloy layers heretofore fabrication have, in many instances, been of less than the optimum quality required for the manufacture of the highest efficiency electronic devices therefrom. Accordingly, because of the limitations imposed by the p-doped semiconductor alloy material, the optimum operational potential of many classes of thin film semiconductor alloy devices have as yet to be achieved.
For example, if a highly transparent, wide band gap, microcrystalline, p-doped semiconductor alloy layer (also referred to as a highly "p-doped layer") could be fabricated, p-i-n and n-i-p type photovoltaic cells manufactured with said microcrystalline, p-doped semiconductor alloy layer would exhibit significant improvement in the operational performance thereof. Such a highly p-doped layer would have a low activation energy and would thus increase the magnitude of the electrical field established across the intrinsic semiconductor alloy layer by the oppositely disposed p-doped layer and n-doped layer, thereby improving the fill factor of the photovoltaic cell. Similarly, the built-in potential of the photovoltaic cell, and consequently, the open circuit voltage generated thereacross would be increased by the presence of the highly p-doped, microcrystalline, semiconductor alloy layer. The improved electrical conductivity of microcrystalline p-doped semiconductor alloy material, vis-a-vis similarly constituted and doped amorphous semiconductor alloy material, would further provide for decreased series resistance encountered by charge carriers in their movement through the photovoltaic cell. The decrease in series resistance would result in improved fill factor and overall efficiency of that photovoltaic cell.
Furthermore, wide band gap, p-doped microcrystalline semiconductor alloy layers are more optically transparent than corresponding amorphous semiconductor alloy layers. Such transparency is desirable, if not essential, in the p-doped layer of a p-i-n type photovoltaic cell because increased transparency allows more light, whether incident upon the p-doped layer or redirected by a back reflector through that p-doped layer, to pass therethrough for absorption in the intrinsic semiconductor alloy layer of the photovoltaic cell. It is in this intrinsic semiconductor alloy layer that charge carrier pairs are most efficiently generated. Therefore, photovoltaic cells employing microcrystalline, wide band gap, p-doped layers of semiconductor alloy material would also produce higher short circuit currents. This consideration of transparency would be especially significant for a tandem p-i-n type photovoltaic device, described hereinabove, which cells are formed of a multiplicity of stacked, individual p-i-n type photovoltaic cells. This is because, in such a tandem photovoltaic device, a light absorbing (narrow band gap) p-doped layer in (1) the upper photovoltaic cell would "shade" one or more of the underlying cells and thus reduce the amount of incident light absorbed in the intrinsic semiconductor alloy layer, the layer with the longest lifetime for charge carriers thereof, and (2) the lower photovoltaic cell would "shade" one or more of the superposed cells and thus reduce the amount of redirected light absorbed in the intrinsic semiconductor alloy layer.
As should thus be apparent, if it would be possible to fabricate a microcrystalline p-doped semiconductor alloy material having a wide band gap, high electrical conductivity and low activation energy, said p-doped semiconductor alloy material would prove to be very beneficial in the manufacture of photovoltaic devices. Similarly, such a p-doped microcrystalline semiconductor alloy material could be advantageously employed in the manufacture of other electronic devices to complement the presently available high conductivity n-type thin film silicon alloy semiconductor material. Obviously, high quality, microcrystalline, p-doped semiconductor alloy materials would have immediate utility in the fabrication of a wide variety of thin film electronic devices such as thin film transistors, diodes, memory arrays and the like. Simply stated, such p-doped microcrystalline semiconductor alloy material could be made to exhibit the high conductivity and wide band gap characteristics of corresponding single crystal semiconductor material and could be made to accept sufficiently high levels of p-dopant material to provide a low activation energy. Further, such p-doped microcrystalline semiconductor alloy materials would not manifest the detrimental properties of large grain boundaries commonly exhibited by polycrystalline semiconductor alloy material; and unlike single crystalline semiconductor alloy materials, they could be produced in a wide range of compositional variations by low cost vapor deposition techniques.
One method for the fabrication of microcrystalline p-doped silicon alloy materials is disclosed by Matsuda, et al in a paper entitled "High-Conductive And Wide Optical-Gap Boron-Doped Si:H Films"published in 1981 by the American Institute of Physics in Tetrahedrally Bonded Amorphous Semiconductors, edited by Street, Biegehem and Knights. As described therein, a glow discharge deposition technique is used for the preparation of thin films of boron doped, hydrogenated microcrystalline silicon alloy material from a gaseous precursor mixture of diborane, silane, and hydrogen under high power, low pressure conditions. The resultant p-doped semiconductor alloy was reported to have an optical gap of 1.8 eV, dark conductivity of about 0.1 ohm.sup.-1 cm.sup.-1, an activation energy of 0.03 eV, and microcrystalline inclusions amounting to 60 volume % in the amorphous network.
While the aforementioned paper of Matsuda, et al discloses a method for the preparation of microcrystalline, p-doped, hydrogenated silicon alloys, said alloys have yet to be optimized for the production of the highest efficency semiconductor devices therefrom. For instance, the conductivity of 0.1 ohm.sup.-1 cm.sup.-1 remains far below (about at least two orders of magnitude below the value that can be theoretically obtained with microcrystalline p-doped hydrogenated silicon alloy material) and the band gap remains narrower than the band gap of the corresponding intrinsic semiconductor alloy material). It is believed that the technique described by Matsuda, et al provides less than optimized semiconductor alloy material insofar as it (1) fails to incorporate fluorine into the host matrix of the semiconductor alloy material, and (2) relies exclusively upon the use of diborane as the gaseous precursor material from which to provide boron for p-doping the semiconductor alloy material.
First, with respect to the use of diborane, while the fabrication of hydrogenated microcrystalline p-doped semiconductor alloy material is a notable achievement, regardless of the gaseous precursor p-dopant material, the polymeric tendencies of a gaseous diborane precursor under the influence of a glow discharge environment would make it desirable to substitute another gaseous precursor source of a p-dopant material. More particularly, (1) diborane is a relatively expensive, toxic, gaseous material which ignites spontaneously upon contact with the ambient atmosphere, thus necessitating the implementation of special production procedures and the use of expensive, specialized gas handling and storage systems, (2) under glow discharge conditions diborane inherently produces semiconductor species exhibiting less than desirable plasma properties. As fully disclosed in U.S. Patent Application Ser. No. 668,435, filed Nov. 5, 1984, of Yang, et al, which application is assigned to the assignee of the instant invention and the disclosure of which is incorporated herein by reference, diborane, under glow discharge deposition conditions, is characterized by a tendency to incorporate polymeric and oligomeric boron species into the depositing host matrix of the semiconductor alloy material, said higher order boron species deleteriously affecting the chemical, optical, and electronic properties of the resultant semiconductor alloy material. Therefore, it would be desirable to be able to fabricate a thin film microcrystalline, p-doped, wide band gap semiconductor alloy material from a gaseous precursor material other than diborane in order to avoid the formation of the polymeric and oligomeric boron species.
Additionally, and as referred to hereinabove, Matsuda, et al fail to incorporate fluorine into the matrix of their thin microcrystalline p-doped semiconductor alloy films. However, as has been shown by the assignee of Applicants' invention, fluorine introduced into thin film semiconductor alloy materials beneficially affects the chemical, electrical, and optical properties thereof so as to render those thin film semiconductor alloy materials more clearly equivalent to corresponding single crystalline semiconductor materials. Therefore, it would also be desirable to incorporate fluorine into the host matrix of the microcrystalline, p-doped semiconductor alloy material so as to gain all of the aforementioned beneficial characteristics of fluorinated amorphous semiconductor alloy materials. In this regard, note that the disclosure of Matsuda, et al is limited to p-doped wide band gap silicon:hydrogen microcrystalline semiconductor alloys. Matsuda, et al do not discuss, describe, or suggest the production of highly-conductive, wide band gap, p-doped, microcrystalline semiconductor alloy materials which incorporate fluorine into the host matrix thereof.
In summary, it may thus be seen that one method of fabricating higher quality thin film electronic devices than are currently available, would be to incorporate fluorine into the host matrix of the thin film, wide band gap, p-doped, microcrystalline semiconductor alloy materials. And in order to fabricate said fluorinated, wide band gap, p-doped, microcrystalline semiconductor alloy material in a commerically feasible manner, it is necessary to either utilize a preexisting process or develop an economical, high volume, preferably continuous process for the fabrication of said alloys, which process is compatible with processes currently employed for the fabrication of the n-doped and intrinsic layers of thin film semiconductor alloy material.
These and other objects and advantages of the instant invention will be apparent from the detailed description of the invention, the brief description of the drawings and the claims which follow.